The present invention relates to vibration damping and has been developed primarily, though not exclusively for the suppression or damping of vibration occurring during a metal rolling process. However, it is to be appreciated that the invention has broader suitability in other applications where it is necessary or desirable to damp or suppress vibration.
Typically strip or sheet metal is rolled to the required thickness by passing the strip between two adjacent rolls which provide the necessary amount of compressive work on both faces of the strip. Generally the principal work rolls are supported in a roll stack by a bearing support means or chocks. The backup roll chocks support backup rolls which contact the respective work rolls in use. A series of pistons are commonly used to apply forces to components of the roll stack, and are typically although not exclusively located in three places:
between the work roll chock and backup roll chock;
between the work roll chocks, and
between the backup roll chocks themselves, these usually being the biggest pistons in the roll stack.
Normally the entire roll stack is supported within a mill frame. Such an adjustable roll support system allows the gauge or thickness of the rolled product to be changed at will by adjusting the vertical position of the chocks and their associated rolls.
Advances in technology have allowed rolling mills to operate at relatively high speeds to increase productivity and efficiency, however the occurrence of mechanical vibrations, generally referred to as chatter in the rolling context, has consequently become more prevalent. Chatter involves roll vibration in a substantially vertical direction in generally large uncontrollable amplitudes of motion at a fundamental frequency. Chatter generally, causes periodic, transverse, bands of light and dark appearance across the rolled strip. In some cases, a matching thickness variation of the rolled strip is associated with the xe2x80x9cchatter bandsxe2x80x9d. Both the banded appearance and the thickness variation are highly undesirable. Not only must the affected product be rejected, but can also result in breakages of the strip during rolling, leading to damage of the mill equipment. Usually it has been found that as mill speeds are increased the vibrations become more severe. Thus, at present, the only reliable remedy for chatter is to reduce the operating speed of the rolling mill which in turn adversely affects mill productivity.
Generally, three distinct types of mill chatter have been identified, namely:
(i) Torsional chatter, which typically occurs in the 5-25 Hz range and causes significant chatter bands across the strip and small thickness fluctuations. This is often referred to as rumble or shudder, reflecting the low frequency range in relation to the audible range of frequencies. The small variations in thickness may cause fluctuations in surface reflectivity which are aesthetically unacceptable;
(ii) Third octave mode chatter, which typically lies in the 125-240 Hz range and produces large thickness variations and strip rupture. It is characterised by a sudden occurrence (usually  less than 5 seconds) and thus appears to be xe2x80x9cself-excitedxe2x80x9d as opposed to externally excited;
(iii) Fifth octave mode chatter, which typically occurs in the range 500-800 Hz and results in transverse banding of the backup and work rolls and matching transverse surface marking of the strip. To cure the problem, an unscheduled backup roll change is often required. There is experimental evidence to suggest that although there is negligible thickness variation, the strip is not flat and has a periodic corrugated waveform. These strip marks (or corrugations) are still visible after the strip has been temper rolled and painted.
Of the three types of chatter vibration, third octave mode chatter is the most destructive and has the most detrimental effects on mill productivity due to the lower rolling speeds required to avoid the phenomena. However fifth octave chatter seems to be more prevalent in rolling mills and is of increasing concern as customers are demanding better surface quality. For each of these types of chatter there is some form of vibration inherent in the roll stack which is associated with the strip chatter marks.
There have been previous attempts to suppress chatter without resorting to decreasing mill speed. Inflatable housing or frame liners have been proposed which increase friction between the chocks and the mill housing or frame in order to inhibit vertical vibrations of the chocks and their associated rolls in the event of chatter. This approach increases friction between the frame and the mill stack and hysteresis may in fact degrade thickness control performance on the mill.
Various other forms of physical vibration damping apparatus are known in the prior art. In SU488635, pairs of co-axial and opposing hydraulic plunger type cylinders are located between the work roll chocks to provide outward thrust to the work rolls in use, to thereby decrease roll overloads when metal strips enter/exit the stands. The end faces of the cylinders contact one another and only the fluid surrounding either cylinder provides any sort of crude shock absorbance. In U.S. Pat. No. 5,730,692 specifically designed rolls themselves function as vibration dampers due to the internal movement of pressurised liquids to absorb shock. In SU570421 pulsating fluid is fed along lines to induce forced vibrations of working rolls which at the correct amplitude and frequency can oppose and cushion the vibration of these rolls in use.
Physical damping units are also known for external positioning on the mill frame. In JP08247211 and in JP05104117 an amplitude sensor detects the vibration frequency of a rolling mill and a xe2x80x9cchatter absorberxe2x80x9d fitted externally to the mill frame itself is activated to counter this chatter.
In U.S. Pat. No. 5,724,846 there is proposed a method whereby a low power vibration component that is non-synchronous is introduced into the mill frame which prevents the rolls from vertically oscillating in any generally large, uncontrollable manner. It is not made clear as to why this system would prevent chatter from occurring and the approach has not been widely used in practice.
In U.S. Pat. No. 5,512,009 a method and apparatus is provided comprising an oscillation-inducing device which is coupled to the roller for inducing an axial oscillation in the roller being of substantially greater frequency than the frequency of rotation of the roller. This method pertains to reducing a specific form of chatter, xe2x80x9coptical chatterxe2x80x9d. Although evidence suggests that this form of chatter is associated with axial oscillations in a grinding mill, it has not yet been demonstrated to be relevant to rolling mills.
In a first aspect the present invention provides vibration damping apparatus for use in a rolling mill, the apparatus including:
a body positionable for sliding movement in an enclosure located at or near a roll chock of the mill; and
damping means integral with the body for providing vibration damping of roll chock(s) within the mill.
By having the damping means integral with the body, the apparatus can replace existing conventional apparatus used to apply forces to the roll stack of a rolling mill and yet still operate so that the occurrence of chatter vibration induces motion within the body which is resisted by damping means which act to dissipate the vibration energy.
Preferably the damping means is a compartment located at one end of the body, the compartment including one or more vibration absorbing components. Preferably the compartment is located at an end of the body outside of the enclosure to abut with one of the roll chocks. Whilst the damping means may be compartmentalised (or part of an enclosure), individual unenclosed damping elements can also be employed such as springs, pads etc mounted at the end of the body. The damping means may also be arranged intermediate the ends of the body.
Preferably an opposing end of the body is located within the enclosure and is in contact with a fluid therewithin, the fluid moveable into or from the enclosure via a passage (eg a tube) ultimately connected to a reservoir, in use to transmit force to the body in the enclosure.
Preferably a second vibration absorbent compartment portion of the body, including further vibration absorbing components, is located at the opposing end of the body and in contact with the fluid.
In an alternative or additional arrangement the second vibration absorbent compartment can solely or additionally define the damping means.
Preferably the enclosure is defined within one of the roll chocks or as part of separate apparatus positionable between opposing roll chocks.
Preferably the apparatus is mounted at a lower of the opposing chocks and an in use uppermost end of the body contacts the underside of an upper of the opposing roll chocks.
Preferably the body is a cylindrically shaped piston.
Preferably the compartment(s) of the body include one or more vibration absorbing components including spring(s) and/or vibration absorbing pad(s). In an alternative arrangement, the second vibration absorbent compartment of the body can include one or more vibration absorbing components including a spring, air etc.
In a further aspect the present invention provides vibration damping apparatus for use in a rolling mill, the apparatus including:
a body positionable for sliding movement in an enclosure located at or near a roll chock of the mill; and
damping means comprising a fluid located within the enclosure and in communication with the body, the fluid being moveable into or from the enclosure such that the fluid movement is responsive to detected vibration to provide for vibration damping of roll chock(s) within the mill.
In a further aspect the present invention provides a method for damping vibration in a rolling mill including the steps of:
(i) positioning a slidable body and associated damping means at or near a roll chock of the mill; and
(ii) regulating the damping means in a manner that provides a counteractive damping to vibration characteristics of the rolling mill.
Preferably the damping means includes springs and/or vibration absorbing pads positionable with respect to the body and step (ii) includes selecting spring and/or pad vibration absorbing characteristics which counteract the vibration characteristics of the rolling mill; or the damping means includes a fluid positionable with respect to the body and step (ii) includes selecting fluid characteristics that counteract the vibration characteristics of the rolling mill.
Preferably the characteristics of the spring and/or pad which are varied include geometry, elastic modulus and damping material constant, and in the case of a fluid, the viscosity and the elastic modulus of the fluid.